In many industries, such as food processing, non-metallic materials are conveyed automatically to various processing machines. A problem arises in such industries because opportunities arise for "tramp" metal to be introduced into the material stream, and such tramp metal would damage the processing machines if conveyed to the machines along with the non-metallic material.
It is known in the art that the non-metallic material stream can be passed through the electromagnetic field of a search coil, and that if any tramp metal is present in the material stream it will disturb the electromagnetic field of the search coil and induce a detectable error signal in the search coil. Responsive to the error signal, the material stream can either be stopped until the metallic material is removed, or diverted to a waste container for a short period of time.
In one type of prior art search coil device, a stream of non-metallic material is directed vertically through the center of a search coil. The coil is connected to circuitry which automatically operates a hinged gate valve to divert the vertical stream of material into a reject container in response to an error signal from the search coil caused by a piece of tramp metal passing through the search coil. However, we have found that when an attempt is made to substantially enclose the entire stream of non-metallic material and the gate valve, the gate valve strikes against the sides of the enclosure and thereby causes vibrations which are transmitted through the enclosure to the search coil. Such vibrations cause disruption of the electromagnetic field of the search coil which can cause an error signal to be generated when no metal has been detected. This leads to a waste of acceptable material.
Another problem with such metal detection systems has been an inability to conveniently test the search coil to determine whether the coil is operating properly. It has been necessary in prior art systems to actually pass a piece of metal through the field of the coil to see whether it is detected. However, when this is done careful steps must be taken to assure that the test metal is removed whether or not it is detected by the search coil. This is a particular problem in vertical pass detectors such as described above, since the piece of test metal must be dropped through the coil and prevented from dropping vertically into a processing machine that could be damaged by the metal.
Still another problem with an enclosed vertical pass metal detector is that an obstruction, such as a large piece of non-metallic material or a piece of tramp metal, can become trapped between the gate valve and the enclosure. This prevents the gate valve from fully returning to its normal position after cycling to divert rejected material, and therefore allows a substantial amount of acceptable material to pass around the obstruction and into the reject container until the obstruction is released by the next cycling of the gate valve responsive to detection of metal by the search coil.